Acute kidney injury (AKI) due to ischemia and reperfusion (IR) is a major unresolved clinical problem with extremely high mortality and health care costs and ischemic AKI frequently complicates major vascular, cardiac and liver surgeries. Although renal tubular inflammation is a major mechanism of ischemic AKI, there is no effective therapy to limit the inflammatory response after renal IR injury impeding effective perioperative clinical care. Peptidyl arginine deiminase-4 (PAD4) catalyzes the post-translational conversion of arginine to citrulline. Our recent novel findings show that that kidney proximal tubule cells not only express functional PAD4 but also that renal IR injury increases the renal tubular PAD4 expression and activity in mice. Furthermore, pharmacological PAD4 inhibition or genetic PAD4 deletion significantly reduced renal tubular inflammation (cytokine/chemokine expression and neutrophil infiltration) and injury after IR. In contrast, recombinant human PAD4 protein exacerbated renal tubular inflammation and injury after IR. However, the mechanisms of PAD4 induction after renal IR, the cell type(s) in the kidney where the PAD4 induction occurs as well as the downstream cellular signaling events generated by renal PAD4 activation remain unknown. Exciting preliminary data suggest that extracellular ATP and its analogs induce renal proximal tubular cell PAD4 by activating P2X7 receptors. This is highly significant as recent studies showed that extracellular ATP released by necrotic cells after IR further promotes inflammation and cell death. Our preliminary data also suggest that PAD4 citrullinates and increases the activity of renal proximal tubular IkB? kinase ? (IKK? also known as NFkB Essential MOdulator or NEMO) to induce pro-inflammatory cytokines via nuclear NFkB translocation. In addition, our preliminary studies suggest that blocking NEMO activation protects against recombinant PAD4-induced exacerbation of ischemic AKI in mice. Based on these preliminary data, our central hypothesis is that extracellular ATP-mediated induction of renal proximal tubular PAD4 exacerbates kidney inflammation and injury after IR via enhanced NEMO citrullination and activation. Furthermore, we hypothesize that inhibiting renal tubular NEMO activation protects against kidney injury and inflammation after IR. We will utilize both in vivo (murine renal IR) and in vitro (proximal tubule cells) models to further elucidate the mechanisms and discover potential therapy for ischemic AKI by testing the following 3 specific aims. Aim #1: To identify the renal cell type and determine the mechanisms of PAD4 induction after ischemic AKI. Aim #2: To elucidate the mechanisms of PAD4-mediated renal inflammation after ischemic AKI. Aim #3: To develop novel therapies for ischemic AKI by selectively targeting proximal tubule NEMO. In summary, our translational approaches will provide a novel understanding of the mechanisms of renal tubular inflammation and injury after renal IR. Our proposal also aims to identify novel therapies to protect against ischemic AKI.